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A study of tropical cyclone structural evolution


The destructive potential of a tropical cyclone is highly dependent on both the intensity and size of the storm. There has been extensive research done on intensity and intensity change, but far less work has focused on tropical cyclone structure and structural changes. The recent highly active Atlantic tropical seasons reemphasize the need for a better understanding of tropical cyclone structural evolution. This is particularly true of the 2005 season which produced a number of storms, such as Katrina, Rita, and Wilma, that not only became extremely intense, but also grew substantially in size during intensification. In contrast to these giants are the storms such as Hurricanes Charley (2004) and Emily (2005), which reached equal intensity, but remained fairly small in size. The goal of this study is to gain a better understanding of what causes these different structural evolutions in tropical cyclones. The inner core (0-200 km) wind-fields of Atlantic and Eastern Pacific tropical cyclones from 1995-2005 from aircraft reconnaissance flight-level data is used to calculate the low-level inner core kinetic energy. An inner core kinetic energy-intensity relationship is defined which describes the general trend of tropical cyclone inner core kinetic energy (KE) with respect to intensity. However, this mean KE/intensity relationship does not define the evolution of an individual storm. The KE deviations from the mean relationship for each storm are used to determine the cases where a storm is experiencing significant structural changes. The evolution of the KE deviations from the mean with respect to intensity indicates that hurricanes generally either grow and weaken or maintain their intensity, or strengthen but do not grow at the same time. The data is sorted by the state of intensification (intensifying, weakening, or maintaining intensity) and structure change (growing or non-growing), defining six sub-groups. The dynamic, thermodynamic, and internal conditions for the storm sub-groups are analyzed with the aid of statistical testing in order to determine what conditions are significantly different for growing versus non-growing storms in each intensification regime. These results reveal that there are two primary types of growth processes. The first is through eyewall replacement cycles, an internally dominated process, and the second via external forcing from the synoptic environment. As a supplement to this study, a new tropical cyclone classification system based on inner core KE is presented as a complement to the Saffir-Simpson hurricane scale.


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Cyclones -- Tropics


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